All plants are members of particular plant families, and these in turn are divided into genera, which vary greatly in size, some containing a single species, others a few thousand. The scientific names of many plants, particularly wild plants, consist of two words; the first part of this name, the genus name, gives the genus the plant belongs to. The first letter of this first part of the name is always capitalised.
For example the genus name for species of oak: Quercus
Genera are then divided into species, the basic unit of plant classification. The second word in a plant’s scientific name provides us with this detail and is known as the specific epiphet, the first letter of which should be in lower case.
Genus name + specific epiphet = Plant name
So for example, the scientific name of the primrose is written:
Primula + vulgaris = Primula vulgaris
Once a genus has been mentioned in text it may be abbreviated to its initial letter when part of a species name (unless this means it might be confused with another genus beginning with the same letter), so for example P. vulgaris.
Euphorbia characia subsp. wulfenii
When wild plants are found growing over a wide geographic area, populations of these plants may acquire slightly different characters. These can be distinguished as subspecies within one species. The word subspecies is usually abbreviated to subsp. but should not be written in italics, although the scientific name of the subspecies is written in italics.
So for example, the scientific name of the subspecies of Mediterranean spurge, Euphorbia characias, native to a region stretching from southern France to Anatolia is:
Euphorbia characias + subsp. wulfenii= Euphorbia characias subsp. wulfenii
Narcissus romieuxii subsp. albidus var zaianicus
Populations and individuals with distinct characteristics found through the geographic range of a species or subspecies may be recognised as varieties (varietas) and forms (forma). These differ from subspecies as they occur throughout the natural range of the species and are seldom correlated with geographical distribution.
Varieties and forms are usually written abbreviated as var. and f., both as lower case and not in italics, although in both cases the scientific name that follows is written in italics.
So for example: Quercus robur + var. haas = Quercus robur var. haas
Quercus robur + f. fastigiata = Quercus robur f. fastigiata
Subspecies, variety and form are ranked in order and a few plants may have a name at each rank.
For example, daffodil : Narcissus romieuxii subsp. albus var. zaianicus f. lutescens
This may prove unwieldy and so it is acceptable to shorten the name; N. romieuxii f. lutescens
NOTE: Very occasionally you may come across a plant with a third name, which has no indication of rank. This is known as a trinomial. Names of this sort are most often seen in old or historic publications; using them was once common practice, but under the Botanical Code these names, if originally published as such, are invalid. However, a few are still found in listings of plant names, usually where the rank of the third name is unknown; this is an unsatisfactory situation and it may be best to treat such names as cultivar epithets until they can be resolved.
+ Laburnocytisus 'Adamii'
Many plants, especially those in cultivation are the result of interbreeding between species, either spontaneously, as in the wild, or through deliberate crossing. These are known as hybrids.
Hybrids between species in the same genus
These are most common and indicated by the use of a multiplication sign. Sometimes hybrids between two particular plants are given a new hybrid name.
For example, the offspring between Quercus robur and Quercus ilex have been named Quercus x turneri.
Not all hybrids have these hybrid names; in this case they are referred to by linking together the names of the parent plants with a multiplication sign. This is known as a hybrid formula.
For example; Quercus robur x Q. macrocarpa
Hybrids between different genera
These are given new hybrid genus names with the multiplication sign written before the new name. So for example, the hybrid between Crataegus and Mespilus is x Crataemespilus.
If more than one species from either genus has been used to make various different crosses, these hybrids are treated as different species.
x Cuprocyparis notabilis (Cupressus arizonica and Xanthocyparis nootkatensis)
x Cuprocyparis ovensii (Cupressus lusitanica and Xanthocyparis nootkatensis)
Where the tissues of two plants have been combined through grafting, resulting in a plant with new characteristics, this is indicated in print by the use of a plus sign before the new hybrid name.
For example, between Laburnum and Cytisus: +Laburnocytisus
Plants with desirable and stable characteristics of flower colour, size, disease resistance and variegation etc., resulting from plant hybridisation and selection, or by natural variation are recognised as cultivars and are given cultivar epithets, to be written in addition to their genus and species names to give the cultivar name. Cultivars should not be called or described as varieties from which they are quite distinct.
The word or words of the cultivar epithet are the final element of the full name and are enclosed in single quotation marks. The first letter of each word is capitalised, and the word or words are never written in italics.
Genus name + specific epithet + Cultivar epithet= Cultivar name
Geranium + clarkei +‘Kashmir White’= Geranium clarkei ‘Kashmir White’
This may be abbreviated to G. clarkei ‘Kashmir White’ once it is established in the text which genus is being discussed, as cultivar epithets should be unique within each genus.
If the cultivar is a hybrid between two different species, the cultivar name does not include a specific epithet unless a botanical name has been provided for hybrids of the appropriate parentage:
(Blue Fertile Group) 'Lingholm'
These are collective names for groups of plants, usually cultivars within a genus, that all have similar characteristics. The word Group (written with a capital letter) is always included as part of the plant name, but not with quotation marks, and never written in italics.
Geranium Rambling Robin Group
If a Group name appears with a cultivar name, it is always enclosed within curved brackets.
For example: Meconopsis (Blue Fertile Group) 'Lingholm'
Disa Kewensis gx 'Alice'
Within orchids, hybrids of the same parentage, regardless of how similar they may be and however many times the cross from which they resulted is made, are given a grex name. Individual plants within this grex can then be selected and accorded cultivar names.
When writing, the lower case letters gx are used after a name to indicate that it is a grex.
Genus name + Grex name + Cultivar name = orchid name
Disa Kewensis gx 'Alice'